Carbs or fuel injection?

A polished Carburettor earlier today...

Its not a competition!  We generally have either one or the other.  Once upon a time, when I was younger, all cars had carburettors.  There was no choice!  There were no fuel injection systems.  (Well there were a few mechanical systems, and specialised race systems but not road cars) 

Carbs work amazingly well, considering that they are just basically a "scent spray" device!  They have had many years of continual refinement. and they evolved into really quite efficient accurate devices.  They have some big advantages over fuel injection, that is of benefit to people that modify cars, or bikes or for people building drag cars / bikes. 

Advantages:

• Cheap, simple to understand and light! (race use)

• Easy to swap some jets to change fuelling characteristics

• No electrics or battery required.  (Some race vehicles have none or only small battery because of weight considerations)

• Work well in most situations

• Can be swapped from one engine/car to another pretty easily

• More instant throttle response is possible than with Fuel Injection as we don't have to wait for sensors to tell a computer what's happening and wait for it to calculate a result!

Disadvantages:

• They can ice up in cool damp weather!

• They do not have as accurate control of fuel / air mixture as modern fuel injection so  can easily kill catalytic Converters...

• Fuel can vaporise or boil in summer making hot restarts difficult.

• The essential for fuel draw "venturi" always restricts airflow slightly reducing max possible power

• Fuel drop out occurs in the manifolds causing higher emissions and fuel mixture dependent on manifold temperature. Fuel sat in the manifold as condensed droplets goes through the engine largely untouched!

Fuel Injection systems are now almost universal on road cars.  The biggest reason for this is the law requiring very tight emission control that usually needs a Catalytic Converter (catatonic diverter, catastrophic inverter or whatever!).  These things are rapidly ruined and become ineffective if used with a normal carb system. A carb cannot keep the fuel mixture so "exact" as fuel injection can.  A slightly rich mixture ruins the Catalytic converter!   The advent of computers makes modern electronic fuel injection possible at a sensible price. 

Advantages:

• Better mixture control

• No "choke" needed for cold starts

• Reliability

• less "flat spots" than carb system

• Higher peak power possible due to no restriction caused by carb venturi

Disadvantages:

• Complexity

• Less flexible to modification of motor

• Re-jetting is impossible and remapping more complex than swapping jets

• weight, battery, computer, fuel pump, regulator etc required.

• harder to find faults

Some of the most popular aftermarket and OEM carb manufacturers are Weber, Dellorto, Holley, SU, Pierburg, Mikuni, Stromberg, Bing, Solex, zenith, Edelbrock, and a good few others.

Some of these companies now make fuel injection systems as well.

For race use or for road use with substantially tuned engines it may be better to use an aftermarket programmable fuel injection system like the Megasquirt, etc, of which there are a rapidly growing number available.

In all cases weather a fuel injection system, or carb, the job is to add the correct amount of fuel that matches the amount of air flowing into the engine to keep the mixture correct!  Modern systems also can and often do control the ignition advance curve and can limit the boost level on turbo's by opening the wastegate as required to match the boost to an internal map..

All content and layout (c) John C Williamson, and respective owners, all rights reserved Aug 2003